It is generally accepted in the CRT display art that CRT (Cathode Ray Tube) display devices display twenty-four or twenty-five lines of information. If graphic information, such as a scenic view of a countryside or a design, is also displayed then in the more popular prior art, two memory systems are used, one for graphics and one for text while in another version of prior art both graphics and text are stored in a bit map memory. When a CRT display device is being used with a data processing system, as a form of output means, it often occurs that the user wants to see the information contents of a document which, for instance, is more than twenty-four or twenty-five lines long. By way of example, an ordinary business letter is often more than twenty-five lines in length. In such situations, it has become the practice to scroll such a document or scroll the contents thereof. That is to say twenty-four lines of a document are shown on a CRT and after a suitable time has elapsed, each top line disappears as the information is jumped, or stepped, upward on the CRT screen, with lines twenty-five, twenty-six, twenty-seven, etc. being added to the bottom of the screen as lines one, two, three, etc. disappear from the top of the screen. Such an operation is known as whole screen scrolling or single region scrolling. In prior art systems, the text is "jumped" (to the viewer) in a deliberate movement off the screen at the top and onto the screen at the bottom of the scrolling region. The "jump" operation occurs because the starting addresses for successive scanning operations are changed by text line values rather than scan line values. In addition the "jump" phenomenon is present because the data bits are moved from one location in memory to another which cannot be accomplished in one frame without the use of elaborate and expensive hardware. The DEC VT 100 effects a form of split screen smooth scrolling but does not employ a bit map memory which enhances the present operation. If the document to be displayed has a fixed section, or fixed sections, as the case may be, and the user wants only to scroll a scrollable section, then such an operation is known as split screen scrolling as mentioned above in connection with the DEC VT 100. An example of such a situation would be where a business letter is being displayed and the letterhead along with the addressor's name and title might be displayed as the upper fixed section. The body of the letter, starting with "Dear Mr. Jones" down through the closing expression could be the scrollable section, while the bottom fixed section of the letter might have the address of the company and a proper telephone number.
While it is possible in the prior art, to split screen scroll text and graphics, it is not possible to split screen scroll graphics with a smooth operation as explained before. In the present system both text and graphics can be split screen smooth scrolled. If a system of the prior art technology were designed to provide split screen smooth scrolling for both graphics and text it would require circuitry to provide two hundred and forty starting address designations (SAD's) or it would require moving the entire contents of a bit map memory in one vertical sync period (which would be economically unfeasible). In the present system there is a maximum requirement of four SADs and four length ending values. The fact that the off screen section of the bit map memory, in the present system, lies adjacent to a scrollable region in the bit map memory enables the present system to add new information, to be displayed, to the off screen region and utilize the new information in a scrolling operation by advancing the scan of the bit map memory into the off screen region under control of the length value parameter. The present invention provides for a split screen smooth scrolling operation with reduced hardware as compared to the prior art.